In conventional semiconductor device assembly, it is known to use adhesive to permanently mount a semiconductor die to a mounting pad or substrate. Typical adhesive die attach processes use curable adhesive, such as epoxy or polyimide, as die attach material to affix the die to a die pad, leadframe, substrate, or socket, for convenience referred to herein generally as the die pad. It is common in the art to dispense die attach material in a controlled amount on a die pad. Die handling equipment used for die attach processes typically employs a pick-and-place tool to lift a die from a wafer tape or other holding mechanism and place it on a die pad. The portion of the die handling equipment that actually makes contact with the die is referred to as a collet. The die is placed on the collet, either by surface contact alone or with assistance from a mechanical ejector pin guiding the die onto the collet. A vacuum force exerted within the collet holds the die in the collet while the tool moves it into the appropriate position for placement on the pre-applied adhesive on the die pad.
Die handling presents technical challenges. Particularly for thin dice, which are becoming increasingly common in the arts, handling during die attach requires great care to avoid cracks or other damage. Some examples of die attach-related failure mechanisms known in the art include backside tool marks, scratches, or microcracks, which can eventually lead to die cracking. Thinner dice are in particular danger from microcracks, which can result from excessive flexing of the die during handling. The contact surfaces of die attach collets are sometimes made from relatively soft plastic or elastomeric materials instead of metal in an effort to avoid causing mechanical damage on the die surface. The practice of using a vacuum to hold the die in the collet is another example of efforts to avoid inflicting damage to fragile dice. The use of a prior art vacuum collet, however, tends to cause a thin die to flex forming a concavity during die placement, which can cause further problems. Damage to the surface of the die can also occur, particularly in the central region, due to contact with the collet.
The amount and distribution of die attach material between the die and the die pad can be crucial to the secure attachment of the die to the die pad and to the long term reliability of the completed assembly. Achieving the appropriate depth and uniform distribution of the die attach adhesive layer, also called the bond line, is a significant challenge. If the bond line is too thin, the bond may be insufficient to hold the die to the die pad. If the bond line is too thick, curing may be inhibited or prolonged, the bond may tend to weaken over time, thermal performance may suffer, or other problems may result. An uneven bond line resulting from non-uniform adhesive distribution can result in similar problems or in a combination of such problems. One of the most threatening problems a non-uniform distribution of adhesive can create is the formation of voids in the adhesive between the die and the die pad. Voids can lead to failures due to insufficient adhesive coverage or thermally induced stresses, for example. The formation of a concavity in the die surface, caused by the flexing of a thin die placed in a vacuum collet common in the arts, can induce these and other problems.
Generally, in addition to the formation of the bond line, a quantity of adhesive is pressed from between the die and the bond pad during die placement. The formation of adhesive that builds from the bond pad to the edges of the die is known as the die attach fillet. The formation of the fillet can be adversely affected by the excess, lack, or non-uniform distribution of adhesive in the formation of the bond line. Excessive die attach fillet may lead to die attach contamination of the die surface. Too little fillet may reduce the strength of the attachment and lead to eventual problems such as die lifting or die cracking. The formation of a proper fillet may be impeded by excessive, inconsistent, or unpredictable die flexion during placement of the die on the adhesive by a die handling collet.
Due to these and other problems, it would be useful and advantageous to provide semiconductor die handling apparatus and manufacturing methods with improved die handling capabilities, particularly for use with relatively thin and delicate dice.